1. Field of the Invention
The present invention relates to glyoxal-crosslinked polysaccharide derivatives, for example cellulose ethers, of reduced unbound glyoxal content, and to a method for decreasing the unbound glyoxal in these polysaccharide derivatives.
2. Brief Description of the Prior Art
In a multiplicity of industrial processes involving polysaccharide derivatives, it is necessary to disperse or emulsify a plurality of components in solid or liquid form in the presence of the polysaccharide derivatives.
However, untreated polysaccharide derivatives can only be dissolved, dispersed or emulsified with great effort in many instances, because when the polymer particles are introduced into an aqueous or water-containing solution of the polysaccharide derivatives, a gel layer forms on the surface of these particles. Since water can only penetrate slowly to reach the interior of the polymer, the dissolution, dispersion or emulsification is achieved with great effort. In addition, the resulting swollen particles which are covered with a gel layer have a tendency to agglomerate, and as such lumps are form thereby and the homogeneous distribution of the remaining components is made possible only by means of time- and energy-consuming mixing operations.
Disclosed and contrasted hereunder are the art-related processes of dissolving, dispersing or emulsifying cellulose ethers in a lump-free manner, for example, by treatment with dialdehydes, such as glyoxal. Examples of processes are cited in U.S. Pat. Nos. 2,879,268, 3,297,583, DE-A-1 719 445 and DE-A-1 518 213. Other polysaccharides can likewise be brought into solution following treatment with glyoxal, for example xanthan gum (described in U.S. Pat. No. 4,041,234).
Also disclosed in the literature cited is a process for delaying solubility of cellulose ether by using glyoxal. When sufficiently large amounts of dialdehyde are used, the treated cellulose ether is initially insoluble in water and only dissolves in a lump-free manner after some time. During this time further components can be added, so that after the complete dissolution of the cellulose ether, a homogeneous solution, dispersion or emulsion is present. The dissolution process can be accelerated by increasing the pH (“Cellulose Ethers”, chapter 3.2.5, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH, Weinheim, Germany).
U.S. Pat. No. 3,356,519, teaches that weak bases, for example, sodium tetraborate are added as additive to glyoxal-crosslinked cellulose ethers to increase the pH of the polymers and thus reduce the time up to complete dissolution of these substances. In contrast with the present invention, the prior art process of accelerating the time for the dissolution, dispersion or emulsion is undesirable and can be suppressed by adding acids. Distinctly, U.S. Pat. No. 3,356,519 teaches that solely cellulose ether powders or cellulose ether granules, are surface-treated with 0.1-0.2% by weight of glyoxal. In contrast, the inventive process relates to water-soluble polymers which are brought to reaction with more than 0.2% by weight of glyoxal. In another contrast with U.S. Pat. No. 3,356,519, the inventively treated polysaccharide derivatives may also be brought into solution in a lump-free manner at the preferred pH in the vicinity of the neutral point. In yet another contrast to U.S. Pat. No. 3,356,519, in the inventive process, the sodium tetraborate is not added in solid form after grinding the cellulose ether. Instead a water-soluble borate in dissolved form is added, preferably before grinding, so that intimate mixing of the methyl cellulose with the water-soluble borate takes place.
Poignantly, according to the teaching of U.S. Pat. No. 3,356,519 (column 2, lines 23-26; Example 1), if sodium tetraborate is added in solid form, no interaction is possible between glyoxal and sodium tetraborate, which interaction is a precondition for achieving the unexpected effects described in this patent.
U.S. Pat. No. 4,400,502 describes contacting anionic water-soluble cellulose ethers with a solution of water, glyoxal and sodium tetraborate in a slurry medium.
U.S. Pat. No. 4,400,502 requires the presence of a non-solvent for the polysaccharide derivative, and a flammable and environmentally harmful organic liquid, which must be removed from the product and reprocessed in a complex manner. In addition, relatively large amounts of sodium tetraborate are required, that is to say at least 50 parts by weight per 100 parts by weight of glyoxal used. It is also of note that this patent does not teach how the procedure is to be followed with alkyl-group-containing polysaccharide derivatives, to reduce the content of unbound glyoxal.
DE-A-2 535 311 describes a process for improving the dispersibility of a cellulose ether in aqueous liquids having a pH greater than 10, by adding boric acid or a water-soluble borate; in addition a dialdehyde, for example glyoxal. Using the process described there, a delayed solubility can also optionally be achieved solely using sodium tetraborate, whereas this is impossible using solely a water-soluble borate or boric acid in the case of the polysaccharide derivatives used according to the present invention. The patent describes treating the cellulose ether with boric acid or sodium tetraborate in a non-acidic, preferably alkaline medium, by setting a pH using one or more additional components. In contrast, by means of the present invention, one can surprisingly achieve a delayed solubility when the cellulose ether is mixed with sodium tetraborate solution containing glyoxal at a neutral pH.
Yet another distinction from the prior art processes is that one cannot infer their effectiveness, as to high retardation of dissolution with low content of unbound glyoxal, on alkyl-group-containing polysaccharide derivatives, which are possibly insoluble in hot water.
As would be realized, like many compounds of the class of dialdehydes, glyoxal, in high amounts, is harmful to health. A reduced content of this compound in cellulose ethers having delayed solubility therefore makes improved handling possible. Furthermore, it would be desirable to keep the unbound glyoxal content in the inventively produced products so low that, in many cases, there would be no requirement to identify these compounds with hazard symbols. As such, the products, because of their thus improved environmental compatibility, can be marketed more successfully. The prior art does not disclose a process for establishing a reduced content of unbound glyoxal in glyoxal-treated polysaccharide derivatives.
It is, therefore, an object of the invention to decrease the content of unbound glyoxal in glyoxal-treated polysaccharide derivatives.